How do I correct input videos that are flipped upside down in blender video edit mode
That's actually pretty easy. In the sequencer editor, select your strip and hit N to display the sidebar on the right. Under Adjust / Transform, check Mirror X and Mirror Y.

However, that will only work for the active strip (the last selected one). If you want to do this with all your strips at once, you can "group" them in a Meta Strip (Ctrl+G), then do the Mirror on that Meta Strip.